1. Field of the Invention
This invention relates generally to tire pressure monitors for vehicles and, more particularly, but not by way of limitation, to tire pressure monitors utilized for signalling a loss of pressure in the tire of a road vehicle.
2. Description of the Prior Art
It is known to provide the wheels of vehicles with tire pressure monitors which detect losses of air pressure in pneumatic tires of such wheels. Examples include the pressure indicator disclosed in U.S. Pat. No. 3,117,195, issued Jan. 7, 1946 to J. L. Woodfill, et al.; the tire pressure warning device disclosed in U.S. Pat. No. 3,588,815, issued June 28, 1971 to Koonce; and the electric pressure device disclosed in U.S. Pat. No. 2,510,785, issued June 6, 1950 to Potts, et al. In general, such devices comprise a pressure transducer mounted on each wheel for detecting a loss in pressure in the pneumatic tire forming a portion of the wheel and a means for transmitting a signal to the passenger compartment of the vehicle in response to the actuation of the pressure transducer in a preselected manner. For example, the indicator disclosed by Woodfill, et al. includes a pressure transducer comprising a cylinder and piston wherein the piston is connected to a lever having an electrical brush disposed thereon. When the pressure in the tire decreases, the lever is pivoted such that the brush contacts a slip ring disposed about the axis of rotation of the wheel.
While the advantages of providing a tire with a pressure monitor are well known, there have been problems in constructing such monitors. The monitor can be complex, and therefore expensive, and the use of the monitor can result in leakage of air from the tire. For example, where a cylinder and piston arrangement are used in the pressure transducer, care must be taken in providing a seal between the piston and cylinder to avoid leakage while still providing sufficient ease of relative movement between the piston and the cylinder such that a small decrease in pressure will result in actuation of the transducer.
It is also important that the pressure transducer be compact such that the transducer can be easily mounted in an out-of-the-way location on the wheel.